1. Field of the Invention
The invention relates to a device and a method for detecting a failure of a low-pressure fuel pump of a jet engine.
2. Description of the Related Art
A jet engine generally comprises a fan, one or more compressor stages, for example a low-pressure compressor and a high-pressure compressor, a combustion chamber, one or more turbine stages, for example a high-pressure turbine and a low-pressure turbine, and an exhaust duct. In the combustion chamber, compressed fuel is injected and burned with the compressed air from the compressors. Each compressor is joined in rotation to a turbine to which it is linked by a shaft, thus forming a high-pressure body and a low-pressure body.
The combustion chamber is fed with fuel by a fuel circuit in which the fuel is driven by fuel pumps, generally a low-pressure pump and a high-pressure pump.
The concepts of low and high pressure for the fuel pumps are totally decorrelated from the concepts of low and high pressure for the bodies of the jet engine, concerning the pressure of different fluids; by convention, and to simplify the description, the acronyms “HP” and “BP” will be used hereinafter in the description to mean “high pressure” and “low pressure” respectively, both for the bodies of the jet engine or their elements and for the fuel pumps.
The HP pump is placed downstream of the BP pump in the fuel circuit. The HP pump thus receives, from the BP pump, fuel compressed a first time and thus having a sufficient pressure to be, after compression by the HP pump, at a sufficient pressure for its combustion with the compressed air; thus, the function of the BP pump is to feed the HP pump with fuel having undergone a first compression.
The BP and HP pumps are conventionally mechanical pumps which can, for example, be “geared” or impelled. A geared pump notably comprises a driving pinion (or wheel) (mounted to rotate on a shaft driven in rotation by a gear system) and a driven pinion (or wheel), the fuel being compressed in a volume between the teeth of the driving and driven pinions. An impelled pump comprises a wheel (or impeller) provided with blades for compressing the fuel and mounted to rotate on a shaft driven in rotation by a gear system, the fuel being compressed by the centrifugal effect during the rotation of the wheel. Conventionally, the BP pump is an impelled pump and the HP pump is a geared pump.
The driving gear systems of the BP and HP pumps are generally driven indirectly by the shaft of the HP body of the jet engine via a power take-off shaft connected to an accessory relay box which links the power take-off shaft to the pumps by a gear system. The accessory relay box is well known to those skilled in the art by the name “Accessory Gear Box” (AGB). It is a box containing a gear system (that is to say, a mechanism comprising toothed wheels or pinions which mesh to transmit a rotational movement of a shaft to one or more other shafts); this gear system is linked to a certain number of equipment items or accessories, such as, for example, an electricity generator, a starter, an alternator, hydraulic pumps, etc., and, as has just been seen, the BP and HP pumps; the gear system transmits to the accessories the movements taken from the shaft of the HP body of the jet engine, in order to drive them.
A failure of the BP pump (for example due to the ingestion of a foreign body becoming blocked between the blades of the wheel of the BP pump and the casing jacketing them) is usually reflected in the breaking of a fusible portion of the shaft driving the wheel of the BP pump, the fuel then passing directly through the wheel without being compressed (the wheel of the BP pump in effect being a free wheel); this is called the “main failure mode” of the pump. In this case, the HP pump is fed with uncompressed fuel and therefore fuel at a pressure lower than the pressure that it would have had in normal operation, which may result in a “cavitation” of the HP pump, namely the suction by the latter of air mixed with fuel; such a cavitation of the HP pump is extremely damaging to its operation and can cause it great mechanical damage even as far as a rupturing of the gear systems driving it in rotation. In the case of rupture, the fuel no longer being driven in the fuel circuit, the combustion chamber is no longer fed with fuel and the engine stops, with all the potential disastrous consequences of such a stoppage of a jet engine in full flight of the aeroplane that it is propelling.
A failure of the BP pump is not detected as such; it is its consequences that are detected, namely the stopping of the jet engine. There is therefore a real danger in the fact that a failure of the BP pump cannot be detected earlier because, the HP pump continuing to be fed, there is a high risk of the abnormal operation continuing until there is a deterioration of the HP pump and stoppage of the jet engine.